1. Field of the Invention
This invention generally relates to a device for hydraulically separating solid fragments according to their densities and, more specifically, to a device for making a density classification of solid fragments by passing the fragments through an upwardly directed flow of liquid.
2. State of the Art
"Wet classification" is a well-known procedure for separating or classifying fragmented solids according to their densities and coarseness. Wet classification devices operate on the principle that coarse particles settle relatively faster in a liquid body than do fine particles of the same specific gravity and, also, that denser particles settle faster than less dense ones. It is well known that if the liquid body is in motion, the differences in settling velocities of the particles becomes more apparent and, thus, sharper size and density separations can be made. In accordance with that knowledge, wet classification devices of the so-called "hydraulic-type" have been developed wherein an upward liquid flow is established and the solids are passed through that flow.
Such classification techniques have been utilized in several fields. For example, hydraulic classification of mineral ore is taught in U.S. Pat. No. 2,250,365 and separation of light gravel from heavier gravel is taught by U.S. Pat. No. 1,949,354. Also, it has been proposed in U.S. Pat. No. 3,682,299 to utilize a rising liquid stream to separate sound gravel from lightweight material. Some of the prior art devices disclose recirculation of the liquid stream.
The prior art devices do not appear, however, to be entirely satisfactory in certain new applications, such as solid waste recycling systems. For such systems to be economical, valuable solid fragments must be effectively separated from worthless solids. For example, wood and plastic and other non-metallic components should be discarded while potentially marketable materials such as aluminum, ferrous metals, copper and zinc should be retained. To accomplish such separations, density classification must be sharper than that which has been heretofore obtained and, if the liquid media is recirculated, provision must be made to avoid clogging of the recirculation means with strings or fibrous strands.